1. Field of the Invention
This invention relates to an anti-skid control system for motor vehicles, which is capable of preventing the wheels of the motor vehicle from skidding during braking operation. More particularly, the present invention is directed to such a system usable with automotive brake equipment having an X-type two-channel system brake oil hydraulic piping arrangement which comprises two, a first and a second, brake oil hydraulic channels arranged in such a manner that anti-skid control with respect to the lefthand front wheel and righthand rear wheel is effected through the first brake oil hydraulic channel while anti-skid control with respect to the righthand front wheel and lefthand rear wheel is effected through the second brake oil hydraulic channel.
2. Description of the Prior Art
Generally, with an anti-skid control system for motor vehicles, anti-skid control is effected by means of microcomputers such that hold valves and decay valves comprising eletromagnetic valves are opened and closed on the basis of electrical signals representing wheel speeds sensed by wheel speed sensors, thereby increasing, holding or reducing the brake pressure, for the purpose of securing improved steering performance and running stability of the motor vehicle, while at the same time shortening the braking distance.
FIG. 1 of the accompanying drawings illustrates, by way of example, manners in which wheel speed Vw, wheel acceleration and deceleration +Vw, -Vw and brake pressure Pw are varied during the operation of the conventional anti-skid control system, together with hold signal HS and decay signal DS for opening and closing hold valves and decay valves.
When the brake equipment of the motor vehicle is not operated while the motor vehicle is running, the hold valves remain open while the decay valves remain closed, and the brake pressure Pw is not increased; and when the brake equipment is operated, the brake pressure Pw is increased at time t1 so that the wheel speed Vw is decreased. A reference wheel speed Vt is set up which is lower by a predetermined amount .DELTA.V than the wheel speed Vw and follows the latter with such a speed difference. More specifically, the reference wheel speed Vt is set up so that when the deceleration (negative accleration) -Vw of the wheel reaches a predetermined threshold level, say -1 G at time t2, the reference wheel speed Vt is thereafter made to linearly decrease with a deceleration gradient of -1 G. At time t3 when the deceleration -Vw of the wheel reaches a predetermined maximum value -Gmax, the hold signal HS is interrupted so that the hold valves are closed, thus holding the brake pressure Pw.
Because of the brake pressure Pw being held, the wheel speed Vw is further decreased. At time t4, the wheel speed Vw and the reference wheel speed Vt become equal to each other, and decay signal DS is generated, by which the decay valves are opened so that reduction of the brake pressure Pw is started. As a result of this reduction of the brake pressure Pw, the wheel speed Vw is changed from deceleration to acceleration, at time t5 when a low peak V1 of the sheel speed Vw occurs. Either at the time t5 or at time t6 when the wheel speed Vw is increased up to the level of a speed Vb that is higher than the low peak by 15% of the difference A between the wheel speed Va occurring at the time t4 when the reduction of the brake pressure is started, and the low-peak speed V1, the decay signal DS is interrupted, and as a result the decay valves are closed so that the reduction of the brake pressure Pw is stopped and thus the brake pressure is held. (In FIG. 1, there is shown the case where the decay signal DS is interrupted at the time t6.) The wheel speed Vw is further increased and a high peak thereof is reached at time t7; thereupon, the brake pressure Pw is again increased. In this case, the buildup of the brake pressure Pw is effected in such a manner that the brake presure Pw is alternately increased and held in succession by the fact that the hold signal is turned on and off mincingly so that the brake pressure Pw is caused to gradually build up. In this way, the wheel speed Vw is decreased, and at time t8, the mode for reduction of the brake pressure occurs again. The brake pressure increasing, holding and reducing modes are effected in combination as mentioned above, and thus the wheel speed Vw can be controlled so that the vehicle speed can be decreased, while the wheels of the motor vehicle are prevented from being locked.
In an attempt to apply the above-described anti-skid control system to a motor vehicle incorporating such an X-type brake oil hydraulic piping arrangement as shown in FIG. 2 wherein the lefthand front wheel and righthand rear wheel are controlled through a first oil hydraulic channel common thereto while the righthand front wheel and lefthand rear wheel are controlled through a second oil hydraulic channel common thereto, it has heretofore been the practice that brake control is effected on the basis of the lower one of the wheel speeds of the two wheels associated with the same brake oil hydraulic channel (referred to as "select low" system hereinafter just for the sake of convenience) or alternatively that the control is effected on the basis of the higher one of such two wheel speeds (referred to "select high" system hereinafter also just for the sake of convenience).
With the "select low" system, however, when the lefthand wheels running road surface and righthand wheels running road surface represent different friction coefficients (referred to as "split friction (.mu.)" hereinafter), the wheels running on the lower friction road surface can be prevented from being locked, while the wheels running on the higher friction road surface cannot be provided with sufficient braking force so that the braking distance tends to increase, since the control is effected on the basis of the rotaional speed of the wheels running on the lower friction road surface. With the "select high" system, on the other hand, since control is effected on the basis of the rotational speed of the wheels running on the higher friction road surface, the braking distance becomes shorter than that with the "select low" system, but the wheels running on the lower friction road surface tend to be locked so that there is the tendency that such a condition that the brake works on the wheels on one side alone, occurs and thus the directional stability of the motor vehicle is lost.